Pathophysiological mechanisms of musculoskeletal disorders

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Esa-Pekka Takala, Finnish Institute of Occupational Health

Introduction

The origin of most musculoskeletal disorders (MSDs) lies in a mismatch between the external load and the capacity of the human body to resist biomechanical and physiological strain. Excessive forces can trigger different pathophysiological processes depending on the tissues affected. It is the magnitude, duration and frequency of loading as well as the time for recovery which determine the physiological effect. Different symptoms in different body sites will depend on how far the physiological processes have advanced. The capacity depends on individual factors such as body build and size, gender, age, and general health, and it weakens with age due to MSDs. However, loading is necessary in order to maintain the capacity and this can be enhanced by an appropriate dosing of the mechanical load.

Variety of symptoms and pathophysiology

MSDs can appear with various symptoms of discomfort like pain, fatigue, muscle weakness, stiffness and limitation of movements, sensory loss and numbness, or local swelling and increased heat due to inflammation. Different body structures (muscles, tendons, joints, bones, nerves) tend to have different pathophysiological mechanisms behind the symptoms.

One common feature for most MSDs is the mismatch between the external load due to physical exertion and posture and the capacity of the human body to withstand that load. In addition to the magnitude of load, the duration and frequency of loading and recovery periods are important. The capacity to resist a load will vary according to individual characteristics (body build and size, gender, age, general health). The capacity varies with time and the human body adapts to the loading (e.g. training effects/ weakening).

Stress – strain concept

Biomechanical approach

Biomechanics refer to the study of mechanical forces affecting the body. Mechanical forces are the most important factors placing stress onto the musculoskeletal system. The muscles produce the forces needed for different activities at work. In addition, gravity acts continuously on the body parts and muscle force is needed to maintain certain body postures.

  • Low forces can evoke high mechanical output, if the lever arms are long. Vice versa, the shorter the lever arm, the more force is needed.
  • Muscles and tendons have short lever arms within the body in comparison to the length of the long bones (arms, legs) for handling the external loads. Therefore the biomechanical forces within the body can be tenfold greater than the external forces. For example, holding a 2 kg weight with arm extended can cause a force corresponding to 60 to 70 kg weight on the small shoulder muscles.
  • The mechanical output of the human body is best in the "neutral" posture of the joints. The local forces acting on the tissues increase at the extreme postures and extreme limits of the joint movements.

Physiological response to loading

Figure 1: Effects of mechanical exposures (loading) on the body (modified from Armstrong, 1993)

In Figure 1[1] physical loading (Exposure) coming from outside the body (EXTERNAL) has physiological effects within the body (box INTERNAL).

  • Individual capacity refers to the characteristics that determine how body tissues can resist external loading and the physiological response to the load. If the biomechanical forces are too high, they will evoke direct injuries in the tissues. The capacity depends on:
    • Body build and size: Strong and large persons can generally resist higher loads than weak and small individuals.
    • Gender: The mean maximal muscle strength of women is about 2/3 of that of men, independent of body size[2][3].
    • Age: Muscle strength grows during adolescence but starts a gradual decrease before the age of 30 years. First this decrease is small but the decline increases with years and is about 8–16% per decade after approximately 50 years of age[2][3].
    • General health: Several diseases can reduce the strength of tissues and slow down the recovery after injuries.
    • Skills: Skilled people can use their body and handle the external forces so that the biomechanical forces within the body will not be too high. Unskilled people are more prone to situations that will cause accidental injuries (e.g. when losing their balance).
  • The dose is the magnitude of loading that will initiate the physiological responses within the body. It is defined by the biomechanical conditions (like posture) and the individual capacity.
  • The physiological response to the dose is usually a cascade process rather than a single response. How far this process goes on determines the final pathophysiological condition.
    • Example of a cascade process: Adenosine triphosphate (ATP) is the primary source of energy needed for muscle contraction. This energy source within muscle cells can be depleted within seconds as the muscle work continues. The cells can produce new ATP by two processes: 1) with oxygen from carbohydrates, fat or proteins (aerobic metabolism) or 2) without oxygen from carbohydrates (anaerobic metabolism). The aerobic production of energy can continue safely for hours. The anaerobic process produces lactate and this can cause harmful reactions, resulting to internal failures within the cells. In tendons, continuous loading can also cause cell damage. If the exposure stops before cellular failure, no further harm will result. If the exposure continues, the damage will continue and the result can be inflammation of the tissues – seen as an acute disorder. If the excessive loading still continues, the result can be chronic inflammation resulting in excessive production of fibrous tissues and pathological morphological changes of the tissues.

Capacity changes over time

The previous example shows how the continuous loading without a sufficient recovery time can result in a condition where the body becomes less resistant to loading. Fatigue during the working shift also reduces the capacity. This reduction can be seen as a reduction in muscular force and endurance. Fatigue also manifests itself as a slowing in neuronal reactions that can cause non-optimal movements in unexpected instantaneous situations (e.g. dropping of heavy objects or injuries due to falls).

Adaptation – slow response to loading

The physiological response to moderate loading is a gradual strengthening of the tissues during rest, if loading is stopped before the development of cellular failure and sufficient time is allowed for recovery. With appropriate loading and rest schedules, the capacity can be improved. Physical training and rehabilitation target this kind of adaptation.

Vice versa, if the loading is continuously on a low level, the body will slowly adapt to this low level with weakening of tissues (e.g. muscles and tendons). The general physical capacity will clearly reduce within a week in bed rest. Immobilisation of joints after injuries will weaken also the bones and can result to osteoporosis and arthrosis.

Variation in work load

Figures 2 a to c describe how the same daily workload can be hazardous under different situations. Although the workload is usually described as mean load of the task, it actually includes high peaks and periods of low load[4].

ERO-10-06-d-3.fig2.jpg
ERO-10-06-d-3.fig4.jpg
ERO-10-06-d-3.fig3.jpg


If the peak loads remain within the capacity limit and the periods of low load are sufficient to allow recovery, then the risk of excessive hazardous loading is minimal (Figure 2a). However, should the capacity become reduced, then the peak load can exceed the limit and evoke some tissue damage, initiating the cascade of responses resulting in MSD (Figure 2b). Due to the fatigue during the shift, peak loads can be hazardous, even though the same peak may be regarded as nonhazardous at the beginning of the shift (Figure 2c).

According to these physiological principles, variations in loading and sufficient recovery periods are mandatory in order to reduce risk of MSDs[5].

Pain

In the evolution of animals, pain has developed to be a sensory alarm system as a way to prevent harmful damage to the tissues. In addition to the neuronal sensation, the perception of pain has an interpretative component. Based on the interpretation, the person can behave so that s/he does not notice the stimulus causing pain and sometimes the cause of pain can result in more damage. The other end of this behavioural spectrum is to avoid all pain, even when there is no hazard of further damage.

Neural pathways

Almost all tissues have nerve endings (receptors) that are sensitive to painful stimuli. These nociceptors react to conditions that are related to tissue injury, such as high mechanical pressure, lack of blood flow (ischemia), extreme heat or cold, or irritating chemicals. The stimulus is transmitted to the spinal cord and a motor reflex will move the body part away from the cause of such a sudden stimulus. The neural stimulus is transmitted to the brain by nerve fibres specific for the pain stimuli; first to the neuron centre called thalamus and then on to several sites within the neural cortex, which are capable of perceiving and analysing the painful stimulus.

The interaction of the numerous neural centres in the spinal cord and brain can modify the sensory signals; they can be intensified or dampened. According to the "gate control" theory other sensory stimuli (touch, pressure, vibration) can reduce the pain stimuli at the spinal level and prevent them from reaching the higher levels of the central nervous system. The neural stimuli from the higher pain centres can also dampen the pain stimuli – e.g. this happens when the pain is treated by hypnosis. On the other hand, if stimuli are received from several neural centres, this can amplify the signals of pain.

If the cause of the pain is massive tissue damage or long lasting inflammation, the nociceptors can become more sensitive to stimuli – even slight pressure can be very painful. This phenomenon is called peripheral hyperalgesia. Sensitization can also occur in the central nervous system so that stimuli other than nociceptive stimuli can be perceived as painful. The chronic pain that can occur after permanent damage to a nerve is an example of central hyperalgesia.

In the normal healing response to tissue damage, the peripheral hyperalgesia is followed by local inhibition of pain after few days. The corrective inflammation process stimulates the nerve fibres to grow up receptors that are sensitive to the encephalins produced by the central nervous system. These chemicals act as "pain killers of the body" and reduce the intensity of the inflammation reaction.

Perception

Perception refers to the interpretation of physical sensations in the light of experience. Subjective feeling of pain includes always some interpretation and individual histories of experiences will vary. Therefore measurement of pain with standard instruments by asking the subject to give a rating will produce extensive individual variation for a similar external stimulus. There is also individual variation in the neuronal response to similar stimulus. However, perception – not the actual pain stimulus – mainly refers to the person's behavioural response to pain.

In the musculoskeletal system, mechanoreceptors are essential since they sense the position of joints and tension in the muscles and tendons. They sense touch, pressure and stretching caused by biomechanical forces. If the forces increase to the limit of potential tissue damage, the pain receptors will gradually become activated. Chemoreceptors sense the chances in the internal chemistry of the tissues. During muscle fatigue, the changes in oxygen reduction as well as changes in other chemicals will be sensed by chemoreceptors even before the nocireceptors are activated. Thus in parallel to the pain signals entering brain, the central nervous system receives signals of other receptors that transmit information on "normal" body reactions; these often are perceived as "discomfort" rather than "pain". For example, in the measures of pressure sensitivity, there are two threshold values: 1) detection of pain and 2) intolerable pain. For most people, the detection of pain measure varies from one measurement to the next.

Physiological responses in tissues

Inflammation

Inflammation is a part of the biological process by which the tissues respond to harmful agents like pathogenic microbes, damage to cells, or harmful chemicals and physical agents (e.g. radiation). Inflammation protects the body from further damage; it not only removes the pathologic agents but it also initiates repair of damaged tissues. If the harmful agents are removed and the repair can continue without further irritation, then recovery can be complete. If the irritation continues, this can result in chronic inflammation with pathological changes developing in the tissues. Sometimes the inflammation can initiate a process that disturbs the immunologic system so that the subject's own tissues are recognized as foreign agents and chronic inflammation targeted to the body's own tissues. Rheumatoid arthritis is an example of this kind of an auto-immune disease affecting the musculoskeletal system.

Depending on the size of the injury and the state of the inflammation process, it proceeds in a cascade-like manner including the vascular and immunologic system. In acute inflammation, the process starts by activation of the immune cells present in all tissues (macrophages, dendritic cells, mast cells, histiocytes, etc.). These cells release substances that stimulate the migration of more immune cells (leucocytes) in the site resulting to the clinical signs of inflammation. The dilatation of local blood vessels causes increased blood flow seen as redness and increased heat. Increased permeability of veins allows blood plasma to pass into the tissues causing swelling. Several chemical substances irritate nerve endings and pain can also be caused by mechanical pressure resulting from the swelling. The final result can be seen as the loss of function.

One outcome of acute inflammation can be the complete healing so that no permanent changes of tissues occur. If the tissue damage is extensive or sometimes due to the specific characteristics of the individual, excessive connective tissue may be formed as fibrosis (scars) and this can disrupt function of the system (e.g. reduction in the movement of the joint or tendon). Sometimes acute inflammation becomes chronic with continuous pain, change of tissue composition and loss of function.

Muscles

Muscles include possess of pain sensitive nerves that can be irritated by several causes. Several different physiological responses can account for the muscle pain related to muscular work[6]. Usually pain in muscles is probably the result of a combination of several mechanisms.

By nature, muscle contraction means increased pressures within the muscles resulting in obstruction of blood flow in the vessels. Lack of blood flow (ischemia) is a potential cause of pain due to static contractions. With high muscular exertions, internal rupture of muscle cells is possible. In addition, the accumulation of Ca++ ions can cause cell damage. Contraction of muscle cells is regulated by the neural system so that the length of the contraction is first set by the muscle spindles that sense the changes in the muscle length. The muscle cells are innervated in groups called motor units. The more that the motor units are activated, the stronger is the muscle contraction. When the energy in a motor unit becomes reduced, it is switched off to allow recovery. It is assumed, that in long lasting static contractions, some muscle fibres may stay active for too long, which may trigger pathologic process leading to pain ("Cinderella hypothesis"). The disturbance in the regulation of muscle spindles could cause local long lasting contractions in parts of muscle – often felt as tender points.

Tendons

If there is excessive or prolonged mechanical loading of tendons, the internal structure of tendons can become damaged. Regeneration involves the infiltration of new vessels and nerves into the tendon, resulting to degeneration[7]. There can be other physiological changes as well; e.g. inflammation of the tendon sheets located on the wrist, shoulder, and ankle (tenosynovitis) or the other connective tissues covering the tendon (paratendinitis). The acute inflammation can progress to a chronic condition, resulting in the formation of fibrosis that can disturb movements. Microscopic ruptures within the tissues are believed to be responsible for the inflammation on the sites where tendons or muscles are inserted to the bones (e.g. epicondylitis)[8]

Joints

The bony surfaces within the joints are covered with cartilage allowing almost frictionless movements between the surfaces. Continuous mechanical friction and accidental injuries can destroy the cartilage. The cartilage of adults has no blood supply and its nutrition is provided by the secretion of fluid by the synovial membrane lining the joint. The recovery of the damaged cartilage is slow and the cartilage layer becomes gradually thinner with age, causing mechanical instability within the joint. Instability increases the biomechanical forces acting on the joint surfaces and the surrounding tissues (joint capsule and ligaments), making them more vulnerable to further injury. The bone reacts to this degeneration with excessive growth and ossification of the ligament insertions. The development of these osteoarthrotic changes is clearly related to the genetic inheritance. Minor injuries within the degenerated joint and the surrounding tissues cause inflammation where the synovial membrane has an active role.

Inflammation of joints can also be the result of microbial infections within the joint or as a reaction to infection in other parts of body (reactive arthritis). In rheumatoid arthritis, the inflammation is off an auto-immune nature.

Nerves

Small capillary vessels supply the nerves along their length. Mechanical pressure blocking the blood supply causes disturbances in the function of the nerve. Short term pressure will appear as numbness, tingling, and loss of sensory and motor function in the area innervated by the nerve. Greater pressure for a longer time can result in more permanent failures. Nerve entrapment refers to conditions where nerves are continuously under mechanical pressure from some other tissues (vertebral disc, bony structures, ligaments, muscles etc.). Severe injury or lesion of the nerve can result in hyperalgesia and neurogenic pain[9].

Musculoskeletal disorders in various body parts

Back

The great mobility of spinal column makes it vulnerable to mechanical disturbances. It is subjected to great biomechanical forces and this can cause local injuries. The bones, muscles, ligaments and the facet joints between the spinal vertebras are well innervated. Local pain can emerge from any of these structures. The intervertebral disc of adults receives no innervation and is pain-free. The disc has no blood vessels and it degenerates with age. Mechanical ruptures of the disc can press down on the local nerves and trigger inflammation. The nerves to the extremities can be entrapped by the bulged disc or in the bony tunnel between the vertebras resulting in radiating symptoms (sciatica).

Neck and shoulder

The neck column can be the source of pathologies similar to those encountered in the low back. Muscles between the spinal column and the shoulder joint are activated during all activities carried out by the upper limbs. Even during light sedentary work, there can be static contraction of muscles that can lead to muscle pain. The trapezius muscle is sensitive to be activated by mental stress so that its activity is greater than that needed to resist the biomechanical forces to maintain the posture.

Due to the long lever arm of the upper limb, the shoulder joint can be exposed to high forces. The tendons around the joint (rotator cuff) have a poor blood supply and are therefore more prone to degenerate with age than the tendons in other locations. Injuries and tendon inflammation can be common causes of MSDs in this area[10]

Distal arm, wrist and hand

Inflammation of tendons and related tissues is the most common pathology occurring in the wrist. In the elbow, the insertion areas of muscles can be sites for epicondylitis[8]. In the forearm, non-specific symptoms are common and probably related to the muscles. Swelling in the carpal tunnel due to any cause (e.g. tendon strain, rheumatoid arthritis, hypothyroidism, pregnancy) increases the pressure on the median nerve resulting in symptoms of carpal tunnel syndrome. The pressure increases in non-neutral postures of the wrist due to biomechanical reasons[9].

Lower limbs

The lower limbs have to bear the weight of the trunk, which imposes high mechanical forces on hips, knees and ankles. The mechanical structure of these joints is more robust than that in the upper limbs. Nonetheless, excessive loading at work will lead to degenerative changes. Osteoarthrosis is the most common cause of symptoms in the hip and knee after middle age. In the ankle, there can be strain on the tendons, although these disorders are more common after sporting activities rather than due to loading at work.

Mental stress and musculoskeletal disorders

Mental stress affects the perception of pain. It also causes a general arousal reaction in the brain, which means that the muscles become activated more than necessary for the intended actions (see 4.2. physiological mechanism in muscles). The arousal reaction can disturb motor control, resulting in an increase of biomechanical loading and increased risk of sudden accidents.

Diseases causing symptoms of musculoskeletal disorders

Several diseases with a specific pathophysiology can give rise to symptoms of MSDs and these symptoms can be worsened due to mechanical loading. In inflammatory rheumatic diseases (e.g. Rheumatoid arthritis, Ancylosing spondylitis), the pathophysiology is attributable to auto-immune reactivity. Some diseases like diabetes or neurologic diseases can worsen the function of nerves making them more vulnerable to mechanical compression. Swelling in the carpal tunnel can increase and make the median nerve more sensible to pressure in diseases with an increased tendency for retention of fluids (e.g. heart failure, kidney diseases, hypothyroidism) or pregnancy.

Red flags

Although most MSDs are related to phenomena related to excessive load and can be handled with conservative treatment, the following pathological conditions do require emergency assessment and treatment (red flags):

  • acute traumatic injuries
  • malignant conditions (cancer)
  • deep bacterial infections
  • rapidly developing spinal compression and nerve entrapments
  • cardiovascular emergency situations (aortal rupture, arterial thrombosis)

References

  1. Armstrong T.J., Buckle P., Fine L.J., Hagberg M., Jonsson B., Kilbom Å., et al., 'A conceptual model for work-related neck and upper-limb musculoskeletal disorders', Scand J Work, Environment and Health; 19: 73-84, 1993.
  2. 2.0 2.1 Stoll T., Huber E., Seifert B., Michel B.A., Stucki G., 'Maximal isometric muscle strength: normative values and gender-specific relation to age', Clin Rheumatol; 19: 105-13, 2000.
  3. 3.0 3.1 Danneskiold-Samsøe B., Bartels E.M., Bulow P.M., Lund H., Stockmarr A., Holm C.C., Wtjen I., Appleyard M. Bliddal H., 'Isokinetic and isometric muscle strength in a healthy population with special reference to age and gender', Acta Physiol (Oxf); 197 Suppl 673: 1-68, 2009.
  4. Takala E.-P., 'Loading of the musculoskeletal system at work' (in Finnish). Työ ja ihminen; 21: 42-57, 2007
  5. Mathiassen S.E., 'Diversity and variation in biomechanical exposure: what is it, and why would we like to know?' Appl Ergon; 37: 419-27, 2006.
  6. Visser B., van Dieen J.H., 'Pathophysiology of upper extremity muscle disorders', J Electromyogr Kinesiol; 16: 1-16, 2006.
  7. Xu Y., Murrell G.A., 'The basic science of tendinopathy', Clin Orthop Relat Res; 466: 1528-38, 2008.
  8. 8.0 8.1 Faro F., Wolf J.M., 'Lateral epicondylitis: review and current concepts', J Hand Surg Am; 32: 1271-9, 2007.
  9. 9.0 9.1 Keir P.J., Rempel D.M., 'Pathomechanics of Peripheral Nerve Loading: Evidence in Carpal Tunnel Syndrome', Journal of Hand Therapy; 18: 259-269, 2005.
  10. Seitz A.L., McClure P.W., Finucane S., Boardman N.D., 3rd, Michener L.A., 'Mechanisms of rotator cuff tendinopathy: intrinsic, extrinsic, or both?' Clin Biomech (Bristol, Avon); 26: 1-12, 2011.


Links for further reading

EU-OSHA – European Agency for Safety and Health at Work (no publishing date available). Musculoskeletal disorders. Retrieved on 30 May 2011, from: http://osha.europa.eu/en/topics/msds

EU-OSHA – European Agency for Safety and Health at Work, Report - Work-related musculoskeletal disorders: prevention report, 2008. Available at: http://osha.europa.eu/en/publications/reports/en_TE8107132ENC.pdf/view

EU-OSHA – European Agency for Safety and Health at Work, Report - Work-related musculoskeletal disorders: Back to work, 2007. Available at: http://osha.europa.eu/en/publications/reports/7807300/view

EU-OSHA – European Agency for Safety and Health at Work, E-fact 45 - Checklist for preventing bad working postures, 2008. Available at: http://osha.europa.eu/en/publications/e-facts/efact45/view

EU-OSHA – European Agency for Safety and Health at Work, E-fact 44 - Checklist for the prevention of manual handling risks, 2008, Available at: http://osha.europa.eu/en/publications/e-facts/efact44/view

EU-OSHA – European Agency for Safety and Health at Work, E-fact 43 - Checklist for preventing WRULDs, 2008. Available at: http://osha.europa.eu/en/publications/e-facts/efact43/view

EU-OSHA – European Agency for Safety and Health at Work, E-fact 42 - Checklist for prevention of lower limb disorders, 2008. Available at: http://osha.europa.eu/en/publications/e-facts/efact42/view

Wikipedia – The Free Encyclopedia, Pain (27 May 2007). Retrieved on 30 May 2011, from: http://en.wikipedia.org/wiki/Pain

Wikipedia – The Free Encyclopedia, Hyperalgesia (15 January 2011). Retrieved on 30 May 2011, from: http://en.wikipedia.org/wiki/Hyperalgesia

Wikipedia – The Free Encyclopedia, Inflammation (7 June 2011). Retrieved on 10 June 2011, from: http://en.wikipedia.org/wiki/Inflammation

Wikipedia – The Free Encyclopedia, Muscle contraction (19 May 2011). Retrieved on 10 June 2011, from: http://en.wikipedia.org/wiki/Muscle_contraction

Wikipedia – The Free Encyclopedia, Nerve injury (19 February 2011). Retrieved on 10 June 2011, from: http://en.wikipedia.org/wiki/Nerve_injury

Wikipedia – The Free Encyclopedia, Tendon (27 May 2011). Retrieved on 10 June 2011, from: http://en.wikipedia.org/wiki/Tendon

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